ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accrued Workers Compensation And Pneumoconiosis Benefits
Workers' compensation expense (benefit)	$ 9,759	$ 7,776	$ (110)
Total expense	14,483	12,411	6,623
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost	3,081	3,424	3,810
Interest cost	2,094	2,262	2,253
Net amortization	(451)	(1,051)	670
Net periodic benefit cost	$ 4,724	$ 4,635	$ 6,733